Earlier glacial melt rate revised downward, but recent melt is accelerating dramatically

Mountain glaciers cover between 500 000 and 600 000 km² of the Earth's surface (around the size of France), which is little compared to the area of the Greenland (1.6 million km²) and Antarctic (12.3 million km²) ice sheets. Despite their small size, mountain glaciers have played a major role in recent sea-level rise due to their rapid melting in response to global climate warming.

Of all the ice-covered regions of the planet, ice loss has been the greatest in Alaska and northwestern Canada, where glaciers cover 90 000 km². Results from the LEGOS glaciologists and their US and Canadian colleagues, published in the February issue of Nature Geoscience, lead them to conclude that these glaciers have contributed 0.12 mm/year to sea-level rise over the period 1962-2006, rather than 0.17 mm/year as previously estimated by a team at the Geophysical Institute at the University of Alaska (Fairbanks). The new estimate was obtained by comparing recent topographies, derived from Spot 5-HRS (SPIRIT project (2) funded by CNES) and ASTER (GLIMS/NASA project), with maps from the 1950-60s, which enabled loss from three quarters of the Alaskan glaciers to be measured.

How did the team from the Geophysical Institute of the University of Alaska estimate that the contribution of these glaciers to sea-level rise was 0.17 mm/year? In 1995, and then again in 2001, the researchers used an airborne laser to measure the surface elevation of 67 glaciers along longitudinal profiles. These elevations were then compared with those mapped in the 1950s and 1960s. From this, the researchers inferred elevation changes and then extrapolated this to other glaciers. Their results, published in Science (3), pointed to a major contribution to sea-level rise for the 1950-1995 period (0.14 mm/year sea-level rise), which then doubled in the recent period (after 1995).

Why did they overestimate ice loss from these glaciers by 50%? The impact of rock debris that covers certain glacier tongues (4) and protects them from solar radiation (and thus from melting) was not taken into account in the previous work. Moreover, their sampling was limited to longitudinal profiles along the center of a few glaciers, which geometrically led to overestimation of ice loss. This new study confirms that the thinning of Alaskan glaciers is very uneven, and shows that it is difficult to sample such complex spatial variability on the basis of a few field measurements or altimetry profiles. Thanks to their regional coverage, satellite data make it possible to improve observations of glacial response to climate change and to specify the contribution of glaciers to sea-level rise.

Ice loss from Alaskan glaciers since 1962 is evidently smaller than previously thought. However, thinning (sometimes over 10 m/year, as in the Columbia glacier) and glacial retreat remain considerable. Moreover, the spectacular acceleration in mass loss since the mid-1990s, corresponding to a contribution of 0.25 to 0.30 mm/year to sea-level rise, is not in question and proves to be a worrying indication of future sea-level rise.

Notes:

(1) from Northern Arizona University (US) and two universities in Canada (University of British Columbia and University of Northern British Columbia).

(2) During the 4th International Polar Year (2007-2009), the glaciologists had free access to SPOT 5-HRS data thanks to the SPIRIT project (SPOT 5 stereoscopic survey of Polar Ice: Reference Images and Topographies). The high-resolution images from this satellite can be used to reconstruct precisely the topography of polar ice and thus study its past and future evolution in response to climate fluctuations. LEGOS is the scientific coordinator for this project, which was carried out with CNES, Spot Image and IGN Espace.

(3) Arendt et al, Rapid wastage of Alaska glaciers and their contribution to rising sea level. Science 297, 382-386 (2002)

(4) The lower parts of a valley glacier.